Acoustic attenuation panels are known for lining the walls of nacelles of aircraft jet engines. Such acoustic structures often are referred to as acoustic liners. Generally, acoustic liners include a cellular core, such as a honeycomb structure, covered on its exterior side by an acoustically resistive front skin, and, on the opposite side, with a reflective back skin. Such a structure is known as a single degree of freedom (SDOF) acoustic liner. Other acoustic liners include a pair of superimposed honeycomb cores separated by a second acoustically resistive layer (or septum), an acoustically resistive front skin, and a reflective back skin, and are known as double degree of freedom (DDOF) liners. Generally, SDOF acoustic liners can be preferable to DDOF acoustic liners because SDOF liners generally are less costly to produce, and are lighter in weight than DDOF liners. Linear SDOF acoustic liners can be preferable because they are capable of attenuating noise across a broader range of frequencies and operating conditions than non-linear SDOF liners.
An acoustically resistive layer is a porous structure that at least partially dissipates acoustic energy by at least partially transforming incident acoustic energy into heat. Often, the acoustically resistive layers used in acoustic liners include continuous thin sheets of material having a plurality of spaced openings or perforations, a sheet of porous layer, or a combination of both. In acoustic liners like those described above, the cells of the honeycomb structure covered by the acoustically resistive face skin form resonant cavities that contribute to the dissipation of incident acoustic energy by canceling acoustic reflected waves and or converting acoustic energy into heat, such as by Helmholtz resonance.
One example of the construction of a prior art SDOF acoustic liner is shown in FIG. 1. In this acoustic liner 10, one face of a honeycomb core 14 is covered by a perforated face sheet 16 having a plurality of spaced openings or perforations extending through its thickness. The opposite face of the core 14 is covered by a non-perforated, reflective back skin 12. The honeycomb core 14, perforated face sheet 16, and back skin 12 can be constructed of aluminum or the like. As also shown in FIG. 1, a fine porous layer 18 extends over the exterior face of the perforated face sheet 16. As an example, the porous layer 18 can be a woven layer such as a fine woven stainless steel layer. The layers 12, 14, 16, 18 of the liner 10 can be bonded together by adhesives of types generally known in the art for composite materials. In this embodiment, the porous layer 18 is positioned on the air-wetted surface of the liner 10.
The SDOF acoustic liner shown in FIG. 1 is of a type known as a linear acoustic liner. Linear liners are liners having acoustically resistive elements that have only a small dependence on the incident sound pressure level (SPL), and typically are characterized by a porous layer 18 like that shown in FIG. 1 that is external to the exterior face of the honeycomb core 14. The fine porous layer 18 provides the liner 10 with increased sound attenuation bandwidth as compared to a liner like that shown in FIG. 1 without a porous layer 18.
A second construction of a prior art SDOF linear acoustic liner 20 is shown in FIG. 2. In this arrangement, the liner 20 also includes a honeycomb core 14, an imperforate reflective back skin 12, a perforate face skin 16, and a porous layer 18. Unlike the linear liner 10 shown in FIG. 1, however, the porous layer 18 is disposed between the exterior face of the honeycomb core 14 and the perforate face sheet 16. In this arrangement, the perforate face skin 16 at least partially shields the porous layer 18 from grazing flow across the exterior face of the liner 20.
Though both of the linear acoustic liners 10, 20 described above can effectively attenuate acoustic energy over relatively wide bandwidths and operating conditions, the porous layer layers 18 of such liners 10, 20 sometimes can at least partially separate from the perforate face sheet 16 and/or honeycomb core 14. For example, the bond between a stainless steel wire layer and an aluminum face sheet or aluminum core may eventually corrode, resulting in unwanted separation of the face sheet from the core. Because such separation of layers is undesirable, there is a need for an improved SDOF linear acoustic liner that is simple in construction, and has enhanced structural durability as compared to the liners 10, 20 described above.